Generally, in an axial flow fan for use as a cooling fan for a rotating electrical machine, there is employed an airfoil—a cross-sectional shape of a blade-on the basis of National Advisory Committee for Aeronautics series airfoil (hereinafter referred to as NACA airfoil or NACA airfoil shape). The NACA airfoil is formed in such a way that a trailing portion thereof is progressively thinner toward a trailing edge thereof; thus, the structural strength of the thin trailing portion is weak, which makes it hard to work or form the portion. For this reason, generally, an airfoil without the thin trailing portion thereof is used.
Japanese Unexamined Patent Publication 2003-74495 (paragraphs 0018 through 0026; FIG. 3) discloses an airfoil having a trailing portion identical with that of the NACA airfoil and comprising an anti-turbulence element while the thickness in the midsection of the airfoil is made thinner than that of the NACA airfoil.
The airfoil as disclosed in Japanese Unexamined Patent Publication 2003-74495 is based upon the NACA airfoil; however, the trailing portion is identical with that of the NACA airfoil, which does not assure the strength of a thin trailing portion or overcome difficulty in working or forming the trailing portion.
Furthermore, in the airfoil in which the thin trailing portion thereof is omitted, an airfoil length—a distance between the leading edge and the trailing edge of the airfoil—is short, which reduces the volume of air stream [flowing along a pressure surface, and a suction surface, of the airfoil], to thereby decrease the pressure differential between both surfaces. A problem has been that a smaller stagger angle for the purpose of compensating for decrease in air stream volume and pressure differential causes reduction of the efficiency of the axial flow fan.
The present invention is directed to overcome the forgoing problems, and an object of the invention is to provide an axial flow fan having an airfoil that improves the efficiency of the axial flow fan and increases the strength and workability/formability of the thin trailing portion.